Semiconductors



Dec; 24, 1957 JENNY 2,817,798

SEMICONDUCTORS Filed May 3', 1954 F21. j 5 A;

IN V EN TOR.

' DIEIRIE'H H. JENNY SEMICONDUCTURS Dietrich A. Jenny, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 3, 1954, Serial No. 427,098

8 Claims. (Cl. 317-235) This invention relates to improved semiconductor materials and to improved rectifying and transistor devices made from these materials. More particularly, the invention relates to improved devices which utilize a germanium-silicon alloy.

Many different semiconductor devices utilizing semiconductive germanium or silicon are known, but the operation of germanium devices is usually subject to relatively severe maximum temperature limitations. Silicon devices have certain electrical disadvantages when compared to those of germanium. The maximum operating temperature of a typical device is determined primarily by the energy gap between the valence band and the conduction band of the semiconductive material of the device. When the temperature of the device is increased to a value at which thermal energy is sufficient to drive substantial numbers of electrons across the energy gap, the semiconductive characteristics of the material are adversely affected. For example, the energy gap of germanium is about 0.71 electron volt and many devices utilizing germanium become substantially inoperative above, temperatures as low as 40 C. On the other hand, silicon has a higher energy gap and its operation as a semiconductor is not adversely affected by much wider normal temperature changes. Silicon, however, is a relatively more diflicult material to prepare in highly pure crystalline form and to fabricate into semiconductor devices such as transistors. Moreover, the semiconductive properties of silicon, such as electron mobility, hole mobility and lifetime are considerably inferior to those of germanium.

In accordance with the present invention, it has been found that the use of alloys, of germanium and silicon permits the design and preparation of useful and practical semiconductor devices of prescribed intermediate temperature and electrical characteristics. The use of such alloys avoids some of the less desirable characteristics of each of the component materials of the alloy. Devices of such alloys thus provide much higher temperature stability normally only afforded by silicon and the greater lifetime and hole and electron mobility normally characteristic of germanium.

It is also known that semiconductor devices made of single crystals of material have certain advantages over devices made of polycrystalline material. Further, in accordance with the present invention, it has been found that single crystal germanium-silicon alloy is suitable for making many different types of semiconductor devices. The alloy may be prepared in polycrystalline form by melting together germanium and silicon. Utilizing a seed crystallite cut from a polycrystalline ingot, a single crystal may be grown from the melt by any of the conventional techniques such as the Czochralski process.

Accordingly, it is an object of the instant invention to provide improved semiconductor devices operable at relatively high temperatures.

Another object of the instant invention is to provide semiconductor rectifying devices having improved electrical and operating temperature characteristics.

it tat Patent 2,817,798 Patented Dec. 24,1957

A further object of this invention is to provide improved semiconductor devices made from alloys of germanium and silicon.

The invention will be described in greater detail in connection with the accompanying drawing in which:

Figure 1 is a schematic, partially cross-sectional, elevational view of one form of semiconductor device having rectifying properties and made in accordance with the present invention.

Figure 2 is a schematic, partially crosssectional, elevational view of another form of semiconductor device having rectifying properties and made in accordancewith the present invention."

Figure 3 is a schematic, partially cross-sectional, elevational view of another form of semiconductor device having point'contact rectifying electrodes and made in accordance with the present invention.

Figure 4 is a schematic, partially cross-sectional, elevational view of another. form of semiconductor device having alloy junction rectifying electrodes and made in accordance with the present invention.

Similar reference characters are applied to similar elements through-out the drawing.

Alloys of polycrystalline germanium and silicon of varying proportions have been prepared by Stohr and Klemrn, Zeitschrift fur Anorg. und Allgemeine Chem. 241, 205-424, June (1939). These alloys were not pre pared so as to be semiconductor materials. in accordance with the instant invention, however, single crystals of germanium-silicon alloy having semiconductor properties are prepared and utilized in the fabrication of rectifying and amplifying type semiconductor devices.

The single crystal wafers for such devices are cut from a single crystal ingot prepared from a melt of germanium and silicon. The single crystal ingot is prepared by taking a seed crystallite, for example, cut from a polycrystalline ingot, dipping this crystallite into the melt and slowly pulling it from the melt according to the Czochralski technique described in Crystal Growth by H. E. Buckley (Wiley, N. Y., 1951). A method of making crystalline semiconducting silicon-germanium alloy having improved uniformity is described in a copending application of Everett R. Johnson, Serial No. 427,357, filed concurrently with the present application, now abandoned, and a method of obtaining large crystalline ingots of uniform predetermined composition of germanium-silicon alloy is described in another copending application of Everett R. Johnson, Serial No. 427,067, also filed concurrently with this application, and now abandoned.

A germanium-silicon alloy suitable for making devices of the present invention, is prepared by adding 1% or more of pure silicon powder to molten germanium which has previously been purified to provide a resistivity of over 1 ohm/cm. The relative amounts of germanium and silicon in the alloy are not critical, but the upper temperature stability limit of the device is increased as the proportion of silicon is increased. From this alloy, as described above, a single crystalline ingot is prepared and Wafers are then cut from the ingot.

Referring now to Figure l, a specific embodiment of a device made in accordance with the present invention comprises a small area electrode type rectifying device wherein a wafer 3 of germanium-silicon alloy is afiixed to a conductive base electrode 4 by a layer of solder 5 to provide an ohmic base connection.

Another electrode 6, preferably in the form of a sharply pointed wire of Phosphor bronze, is supported in contact with another surface of the alloy body at a point remote from the base support electrode 4. The electrode 6 is a rectifying electrode. The surface of the alloy crystal in contact with the rectifying electrode 6 preferably is ground with A1 0 and etched according to the method hole and electron mobility as compared with devices .of

pure silicon, thus providing the preferredproperties of each component element.

Figure 2 illustrates a conventional alloy junction rectifier type semiconductor device according to the invention. This device is made from a. wafer 3" of'semiconductive germaniumsilicon,. alloy, ,for example, ofP-v type conductivity, which is dipped into an-etch solution consisting of:

Part 70%HNO s. 52% HZFZ H2O 1 The lower surface of the etched wafer is :soldered to a;con-, ductive base electrode 4 by a layer of solder. 5 to form an ohmic base connection. A small pellet 8 of a mate-.

rial such as antimony, capable of. imparting. N-type conductivity to semi-conducting germanium-.01: silicon, is al-. loyed to the upper surface 9 of. the germanium-silicon alloy wafer 3. The pellet8 of antimony is, placed on the surface 9 of the etched wafer at room temperature and is heated for 5 to 10 minutes at 600 to700 C; to form a,

rectifying junction 10. After cooling to room. tempera? ture the alloy is etched in two stages without intermediate, washing to clean the area around and improve theelec trical characteristics of thejunction 10,.and to improve;

the electrical characteristics of thegermanium-silicon alloy surface. The first etch solution. preferably con.-

An electrode lead 11 is affixed to the alloyed pellet. The device of Figure 2 thus comprises a junction-type rectifier. Any other material, such as bismuth or vanadium, capableof imparting N-type conductivity to germanium or silicon, may be substituted for antimony.

If the body of the wafer 3 is of N-type conductivity, the pellet 8 should be of aluminum, gallium, indium or;

boron, or other material capable of imparting P-type con-.-

ductivity to the semiconductor.

The specific embodiments described include only single rectifying elements providing diode operation. It should be understood that the device of Fig. 1 may be-modified' as in Fig. 3 to include an additional rectifying point contact electrode 12 to provide a transistor. In this transistor 5 device the electrode 14, soldered to the wafer 3, functions as a base electrode.

Similarly, the device of Fig.2 may be modified, for example as shown in Fig. 4; to include an additional rectifying junction 13 and'an electrode lead 16 to provide a transistor device. This device also includes a base electrode which may be'in the form of a nickel tab soldered to the wafer 3.

There have thus been described; improved semiconductive materials and diode and transistor devices utilizing:

15 an alloy of germanium and silicon.

What is claimed is:

l. A semiconductor device including in combination a single crystal body of an alloy of germanium and at least 1% silicon having semiconductingproperties and at least one rectifyingelectrode connected thereto.

2. A deviceaccording to claim 1 in whichsaid rectifyingelectrodeis asmall area type electrode.

3. A semiconductor device comprising in combination a single crystal waferof germanium-silicon semiconducting-alloy ofone conductivity type and at least one P-N junction rectifying electrode.

4; A semiconductor device including in'cornbination a single crystal body of an alloy of germanium and at least 1% silicon having semiconducting properties and a plurality ofrectifying electrodes connected thereto.

5. A semiconductordevice of the alloy rectifying junction type comprising in combination a single crystalline wafer of germanium-silicon semiconducting alloy of one conductivity-type and a plurality of alloy junction type rectifying electrodes.

6. A semiconductor device including in combination a single crystal body of an alloy of germanium and at least 1% silicon having semiconducting properties and at least onelarge area rectifying electrode in contact therewith.

7. A semiconductor device including in combination a single crystal body of an alloy of germanium and at least 1% silicon having semiconducting properties and a plurality of large area rectifying electrodes in contact therewith.

8. A transistor device comprising a semiconductor body of single crystal alloy of germanium and silicon and emitter, collector, andbase electrodes connected thereto.

References Cited in the file of this patent OTHER REFERENCES General and Physical Chem, Chem. Abstracts, vol. 33: 

